This invention relates to a process for reducing the quantity of hydrolyzable chlorides and other impurities in isocyanate compounds, particularly isocyanate compounds which are prepared in the reaction of an amine with phosgene.
Many isocyanate compounds are commercially prepared by reacting the corresponding amine with phosgene. Although this process proceeds in good yield, it is plagued by the formation of a significant amount of undesired by-products. These by-products, which include tars, unreacted phosgene, HCl, and other chloride-containing species, must be removed from the isocyanate prior to its use. Their presence is undesirable due to the corrosive nature of chlorine, and because chlorine interferes with catalysts which are used in preparing polyurethanes. This removal is commonly done on a commercial scale by refluxing or heat treating the crude isocyanate compound to drive off the volatile impurities.
It is known to add gaseous HCl or other polar gas during the reflux step in order to accelerate the removal of volatile impurities. However, even using these gases, the removal of volatile impurities is not accomplished as rapidly or as completely as desired. In addition, this method requires the use of additional reagents as well as storage equipment and apparatus for metering the gas to the process stream.
Accordingly, it would be desirable to provide a process for reducing the quantity of hydrolyzable chloride impurities in isocyanate compounds, which process more rapidly and effectively accomplishes such removal.